As the process for producing polyamide by polycondensation of a dicarboxylic acid component and a diamine component, in general, a process in which an aqueous solution of a nylon salt is obtained from the dicarboxylic acid component and the diamine component and, then, melt polymerization is conducted under pressure, is widely known.
A process in which a dicarboxylic acid component and a diamine component are directly melt polymerized in the absence of solvents without preparing an aqueous solution of a nylon salt is disclosed (JP 57(1982)-200420A). In this process, the diamine component is added while the temperature of the formed polymer is elevated at the melting point of the polymer or higher, and this process is economically advantageous since removal of water (water used in the aqueous solution of a nylon salt) and solvents by distillation is not necessary.
In this process, it is advantageous from the standpoint of the production that the boiling point of the diamine component is the same as or higher than the melting point of the formed polyamide. When the boiling point of the diamine component is lower than the melting point of the polyamide, the polycondensation does not proceed efficiently due to vaporization of the added diamine. The boiling point of xylylenediamine is about 274° C., which is relatively higher than boiling points of other diamines conventionally used for production of polyamides. For example, hexamethylenediamine has a boiling point of 199 to 205° C. Therefore, the process comprising directly melt polymerizing a dicarboxylic acid component and a diamine component in the absence of solvents is an advantageous process when xylylenediamine is used.
On the other hand, when a polyamide is produced using a reaction apparatus of the batch-type, portions of the polyamide and oligomers are left remaining after the polyamide is discharged from the reaction apparatus. Since, in general, these substances have great viscosities, these substances are attached to the inner wall of the apparatus, stirrer shafts and rotating shafts and remain in the reaction apparatus. In general, these substances are left remaining in the form of thin films and does not cause serious problems as long as these substances remain in this condition.
However, when the content of para-xylylenediamine in xylylenediamine is 20% by mole or greater, substances attached in the block form are formed in the area in the vicinity of the interface of the gas and the liquid. This phenomenon arises since the formed polyamide exhibits a higher temperature of crystallization and a greater rate of crystallization as the content of para-xylylenediamine in the xylylenediamine is increased, and this change in the properties enhances the attachment. This phenomenon arises more markedly when the content of para-xylylenediamine is 30% by mole or greater. The polyamide attached in the area in the vicinity of the interface of the gas and the liquid is cooled in the gas phase area and tends to be crystallized, and the crystallization proceeds rapidly due to the great rate of crystallization. Even when the crystallized polyamide is heated at a temperature of the melting point or higher, re-melting thereof is more difficult than that of the polyamide in the amorphous condition, and the attachment proceeds more markedly.
The attachment described above tends to take place, in particular, at stirring shafts and rotating shafts since the temperature at these places is lower than at other places and the flow of the polymer tends to stagnates. The substances attached to stirring shafts and rotating shafts gradually grow by being coated with the polymer and form attached substances in the block form. This attached substances are subjected to heat history, and occasionally, the degree of polymerization is increased, and gels are formed.
The attached substances in the block form is occasionally peeled off during the production of the polyamide and mixed into the obtained product as white unmelted substances to deteriorate the quality of the product. The unmelted substances cause problems such as clogging of die holes during discharge of the polyamide from the reaction apparatus and interruption of strand formation, and the stable pelletizing operation is disturbed. Therefore, it is necessary that the reaction apparatus is periodically opened, and the attached substances are manually removed or washed off with solvents.
As the apparatus for decreasing the attached substances in an apparatus, a stirring blade which has a simple structure, and suppresses attachment of polymers to wall surfaces of a vessel and formation of blocks, as well as enables to operate for a long time with stability and a mixing apparatus equipped with the stirring blade are disclosed (JP 9(1997)-313912A). However, no descriptions can be found at all in the document on the attachment to stirring shafts or on the structure or the circumferential speed for suppressing the attachment to stirring shafts although the attachment to wall surfaces of a vessel is mentioned. As for the polymer for the treatment, no descriptions can be found at all on polyamides.
As the process for preventing attachment of a polymer to the inner wall of the reaction apparatus and a stirring shaft in the area in the vicinity of the interface of the gas and the liquid using a simple means, a process in which the liquid phase portion and the gas phase portion are stirred by stirring blades and the gas phase portion is cooled in the polymerization of a polymerizable monomer in a reaction apparatus of the stirred vessel type, is disclosed (JP 10(1998)-158307A). However, in this method, it is described that the process is a process for producing methacrylic polymers, and no descriptions can be found on polyamides. The stirring apparatus is not particularly limited as long as the apparatus can stir the surface of the reaction fluid and the gas phase portion, and no descriptions can be found on the shape.
Under the above circumstances, when a polyamide is produced by directly melt polymerizing a dicarboxylic acid component and a diamine component in the absence of solvents using xylylenediamine comprising 20% by mole or more of para-xylylenediamine, a process in which attachment of substances in the block form to a stirring apparatus is suppressed to increase the efficiency of the production and to decrease contamination of the obtained product with unmelted substances derived from the attached substances, has been desired.